Same thing happened to me and I changed the radiator, then it all happened again! It was the intake manifold and head gaskets that were the problem all along. Get a pressure check done, or if you know how and have the equipment, do it yourself. Another way to check is if you see bubbles rising in your radiator while the engine is running and/or you have white smoke coming out of your tailpipe, plus you are losing coolant but can't find a leak anywhere - this means it is being sucked into the engine and being burnt off!
